Such a transmission is generally known and is for example described in EP-A-1 579 127. In this known transmission the said normal force is actively controlled by a control system of the transmission based on a difference between an actual transmission slip value and a desired slip value, i.e. by adjusting the normal force such that the said difference is minimised. In this respect, it is recalled that the term slip is used to refer to the difference in (tangential) speed of the transmission components in the said frictional contact.
Several methods are available to determine and/or measure the actual transmission slip. In practice, in calculating the actual slip in the belt CVT, in addition to the (longitudinal) belt speed also a running radius thereof at each pulley, i.e. between pulley discs thereof, is to be taken into account in order to determine the local tangential speed of the pulley. Moreover, the belt CVT actually comprises a series arrangement of two frictional contacts, i.e. one between the drive belt and each pulley, which should both be taken into account. In this case it is convenient to define and determine the transmission slip in relation to the deviation between the transmission's speed ratio, i.e. the difference between or quotient of the rotational speed of the respective pulleys/shafts, and its geometric ratio, i.e. the difference between or quotient of the running radius of the drive belt on the respective pulleys.
Besides the instantaneously prevailing or actual transmission slip, the known control method also requires a desired value for the transmission slip for the control of the normal forces. Indeed several publications are available that address this issue, e.g. by providing a method for selecting a desired slip value in dependence on the transmission ratio and/or the torque to be transmitted. In this respect it is as an example referred to EP-A-1 526 309 that a/o suggests to adopt as the desired slip value the pre-determined amount of transmission slip that provides the optimum torque transmission efficiency.
Although the known control method may function well per se, it can be difficult to implement in practice at least in mass production. First of all, it is difficult to measure the transmission geometric ratio, as determined by the running radii of the drive belt on the pulleys, sufficiently accurate. Moreover, the known means and/or computation methods for determining the actual transmission slip will add to the manufacturing cost of the CVT unit and thus to its economically viable sales price, whereas the increase in vehicle fuel efficiency and cost savings made possible thereby occur and accumulate only during the—prolonged—use of the vehicle. Any such initial investment typically inhibits consumer acceptance and thus hampers the introduction of this new technology. Further, the known control method requires a relatively complicated algorithm that is able to generate an actual transmission slip value, to generate a desired slip value in dependence on the operation conditions of the CVT, to compare both said slip values and to generate appropriate control signals based thereon, all in real time and under unpredictably varying circumstances, e.g. disturbances. It has been found difficult to design such an algorithm that is sufficiently flexible and still provides the required computational speed and robustness.